1. Field of the Invention
The present invention relates to a semiconductor wafer and a semiconductor device including a nitride semiconductor layer.
2. Description of the Related Art
Generally, a nitride semiconductor layer is formed on an inexpensive silicon substrate or sapphire substrate. However, the lattice constants of such semiconductor substrates and that of a nitride semiconductor layer are greatly different from each other, and coefficients of thermal expansion thereof also differ from each other. Therefore, a large strain is generated in a nitride semiconductor layer formed by epitaxial growth on a sapphire substrate or a silicon substrate. As a result, the nitride semiconductor device is susceptible to cracks or degradation of the crystal quality. Compared to a sapphire substrate, a crack is especially more serious problem for a silicon substrate because of a sizable lattice mismatch with a nitride semiconductor layer.
In order to have a conductive nitride semiconductor layer, impurity doping into the nitride semiconductor layer is generally used for generating conducting carrier.
However, the crystal of the nitride semiconductor layer is hardened due to the impurity doping, making it more likely to have cracks. Thus, impurity doping into a nitride semiconductor layer formed on a silicon substrate or a sapphire substrate makes designing of devices more difficult due to crystal hardening and degradation of crystal quality.
Moreover, in order to improve element characteristics, it is necessary to increase the film thickness of a semiconductor functional layer such as a nitride semiconductor layer which forms hetero junction where an operating current flows or a nitride semiconductor layer which has a light-emitting function. By increasing the film thickness of the semiconductor functional layer, the crystal quality thereof is improved. Having said that, to increase the film thickness, it is also necessary to increase the film thickness of a buffer layer arranged between the semiconductor functional layer and the semiconductor substrate. Thus, cracks become more serious problems.